If you're a single user, the usage you describe should be comfortably delivered by a 50 Mbps connection. Here's my thoughts to getting a good deal:
Retentions staff are paid to do a job, and that's to get customers back on a fixed term contract at the minimum discount. They do this day in day out, they're trained in how to handle negotiation, and they're paid incentives to get the right outcome for VM. You or I negotiate a new broadband deal once every year and a half, so unless your day job is in procurement, then you need to make up for the unequal skills.
First thing, loyalty counts for nothing. There's no emotion here, no obligation, no need to offer you a good deal simply because you'd like that, or you think you deserve it. The agent will deal with perhaps fifty or sixty callers EACH DAY, every one phoning up and saying, "I'm a loyal customer, please can I have some money off?" That's water off a duck's back to them.
Second thing, your only leverage is the threat of cancelling and the offers you could get elsewhere (which the agent may already know, or ask for details of). It also matters that the agent believes you would cancel. Out of those sixty callers, at least half will be saying they want to cancel, some mean it, but many don't. If you really have no intention of cancelling then you need to be good at acting it out, because the agent will attend regular refresher courses on identifying customers who have no intention of leaving. If they can keep a customer yet offer £3 a month less discount than they have the leeway to, that's £54 marginal value over the life of the 18 month contract, for say 9 minutes work. If you think about how that adds up across the roughly 1.8 million calls put through retentions each year you'll see why it is worth VM acting in this way.
The agent you are dealing with will probably be paid a bonus if they can retain you, and works from a fixed discount pot in each sales period. Which means what they'll offer varies by agent, by how much of the pot they have left, and what the pot is set at in the first place (none of which we can know). The agent won't be paid the bonus if you end the call with "I'd like to think about that". If you end the call and phone back, it's a new agent, they have no reason to offer you a better deal, and because you're phoning a second time they have reason to believe you're a bit invested in staying with VM. if you get offered a good deal you have to accept it then and there - know what you'd pay as a customer elsewhere and keep that in mind.
The agent will usually play on the speed advantage VM have for most customers. You should acknowledge that, but don't let it become the lynchpin of an argument why you need to pay more than competitors - always bring any discussion back to what you see as poor value from VM relative to competitor offers "yeah, the speeds great, but it's still the total cost that puts me off - if I go with XXXX that'll save me £17 a month, and over the contract period of 18 months that's over X hundred quid difference, I can't afford to ignore that".
Always, keep it polite and cheerful, the agent is doing their job, and you're playing a game. They know that, you know that, but one of the rules is neither side allude to the fact that it's all for show, and all that matters is the best price they'll offer you against your clear message that you'll cancel if the price isn't right. So the first offer they make can almost always be bettered. A bit of human engagement at the start of the call often helps - eg acknowledging that they're working a weekend, or asking if that's a Geordie accent I hear? type of thing. If you don't think the agent is moving in the right direction, then politely terminate the call, and ring back another day, bearing in mind the next agent will know you're calling a repeat time.
Sometimes VM won't offer an acceptable discount, the customer cancels, and then during the notice period they get an outbound retentions call offering an irresistible price. That's great if it happens, but you cannot rely on it happening, so only cancel if you really will take your business elsewhere.
Somebody always complains "it's not fair, they should offer everybody their best price", and I'll note here that isn't how the world works. VM don't want to offer lower prices across the board, and if they lowered standard prices they'd initiate a damaging price war with BT, who are larger and would win. It's a free market, if people don't like the price and package then they have the option to take their business to a company they prefer. That's how markets do and should work.